https://www.nsa.gov/news-features/declassified-documents/venona/
The U.S. Army's Signal Intelligence Service, the precursor to the National Security Agency, began a secret program in February 1943 later codenamed VENONA
The VENONA files are most famous for exposing Julius (code named LIBERAL) and Ethel Rosenberg and help give indisputable evidence of their involvement with the Soviet spy ring.